In recent years, cathode ray tubes for visual display indication have often been employed for the control or monitoring of terminal equipment of a computer or a plant system wherein the display indication is usually a numeral, alphabetic figures, katakara (i.e., the square Japanese syllabary or figure), etc. For this purpose, a color cathode ray tube is particularly advantageous because, from a functional standpoint, it has a great capacity for storing and visualizing a wide variety of information. Also, the display indication can easily be read.
However, some conventional cathode ray tubes are designed to primarily display continuously moving pictures, and therefore, it is desired that afterglows of the respective phosphors emitting different colors are short enough so that they are invisible to the naked eye.
Where, on the other hand, the cathode ray tube for display indication is primarily intended to display letters, characters or figures which are still for a certain period of time (as opposed to continuously moving pictures), it is desirable and beneficial to employ a long-afterglow type phosphor, for several reasons. For example, by employing a phosphor having an adequate long-afterglow, it is possible to reduce the so-called "flickering" of the display image so that the image can more clearly be seen. Further, it is possible to reduce the overall number of picture elements to be transmitted to the screen for display. This is advantageous for two basic reasons: it creates less eye strain for the image reader, and chances of misreading the displayed image will be reduced. Further, the circuit can advantageously be simplified when using a long-afterglow type phosphor.
Representative phosphors currently employed for such a purpose include a green emitting Zn.sub.2 SiO.sub.4 :Mn,As phosphor, and a red emitting (Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn phosphor. The former has B.sub.10% (reduction time of luminance brightness to 10% after cessation of excitation) of 150 msec, while the latter has B.sub.10% of 120 msec. Thus both of these phosphors have satisfactory long afterglow characteristics.
However, the above-mentioned conventional long-afterglow phosphors generally exhibit somewhat poor emission efficiency. Particularly, as the spectral luminous efficacy of the red phosphor is relatively low, its luminance brightness is insufficient from a practical standpoint as compared with the green phosphor. In order to increase the luminance brightness thereof, a short-afterglow red phosphor having high luminance brightness (such as may be used for a color cathode ray tube for a television, for instance), Y.sub.2 O.sub.2 S:Eu, is combined with the long-afterglow red phosphor. This technique is described in Japanese Pat. No. 917125. However, as the afterglow is as short as B.sub.10% =2 msec for such a short-afterglow red phosphor, the amount thereof that can be combined with the long-afterglow red phosphor is somewhat limited, and sufficient practical emission efficiency still can not be obtained in accordance with this type of "hybrid" long-short afterglow type phosphor.
U.S. Pat. No. 3,457,184 discloses europium-activated yttrium oxide phosphors which are prepared by a two-stage firing process. First, precipitated compounds of yttrium with europium are heat decomposed to oxides by firing at 600.degree.-1200.degree. C. Then, at least one flux, such as barium fluoride, sodium fluoride, borax, etc., is added within a specific amount, and this mixture is further fired at 1000.degree.-1400.degree. C. However, the emission characteristics of this type of phosphor could still be improved from a commercial view.
More recently, a long-afterglow red phosphor having high luminance comprising a tervalent europium-activated yttrium oxide of the formula Y.sub.2 O.sub.3 :Eu containing at least one member selected from the group consisting of barium fluoride and magnesium fluoride, has been proposed as described in Japanese patent application (OPI) No. 61-266488 laid-open Dec. 26, 1986 (the term "OPI" as used herein refers to a published, unexamined patent application), corresponding to U.S. patent application Ser. No. 06/867,854 filed May 21, 1986. However, there is a problem in that the color tone of the phosphors described therein, such as a Y.sub.2 O.sub.3 :Eu.BaF.sub.2 phosphor, for example, is undesirably shifted to the orange side, as compared with a Y.sub.2 O.sub.2 S:Eu short-afterglow red phosphor. Therefore, room for improvement in the spectral emission characteristics of long-afterglow phosphors clearly exists.